1. Field of the Invention
The present invention relates to heating apparatus and particularly to heating apparatus that obtains heat by circulating water through a heat exchanger in the flue of an independent combustion heat source.
2. Description of the Prior Art
A common hot water heater of some years back used a storage tank with a separate gas-fired combustion unit containing a helix of copper tubing. The bottom of the tank was connected to the bottom of the helix while the top of the helix was connected to the top of the tank. Cold water from the bottom of the tank would rise through the helix as it was heated by the gas flame and pass into the top of the tank. The gas would be turned off when a predetermined temperature was reached. Today domestic hot water heaters are most commonly one of three types: Storage tank with integral gas burner, storage tank with integral electric heating elements and heat exchanger connected into a central heating furnace of the water or steam types with or without storage tank.
Domestic hot water heaters operating off central heating furnaces conventionally absorb their heat from the water in the furnace rather than directly from the flue gasses. This is necessary so that the domestic hot water does not reach a dangerously high temperature while at the same time avoiding the necessity of turning off the furnace merely because the domestic hot water is too hot. This is the same reason that domestic hot water is seldom drawn from hot air furnaces. The hot air readily rises far above the boiling point of water and would require shutting down the central heating system when the domestic hot water approached a hazardous temperature level.
Besides the above limitations, it has always been a disadvantage of using a central heating furnace for domestic hot water that in the summer it results in operating an inefficiently large unit for providing a small amount of heat. On the other hand, in cold weather when the central heating furnace is functioning anyway, the central heating furnace becomes a much cheaper and more efficient source than the electricity or gas integral units in integrated hot water heaters.